Growth of solid conical structures during multistage drying of sessile poly(ethylene oxide) droplets

TL;DR

This study investigates the formation and growth of conical structures in drying poly(ethylene oxide) droplets, revealing a multi-stage process that challenges existing models and highlights the role of PEO's unique properties.

Contribution

The paper introduces a new four-stage model for droplet drying and conical structure formation, demonstrating that surface area increases during growth, contrary to previous skin-buckling explanations.

Findings

Conical structures grow during drying with increasing surface area.

A four-stage drying process is proposed, including bootstrap growth.

Model predictions align with experimental observations.

Abstract

Sessile droplets of aqueous poly(ethylene oxide) solution, with average molecular weight of 100 kDa, are monitored during evaporative drying at ambient conditions over a range of initial concentrations $c_0$. For all droplets with $c_0 \geq 3%$, central conical structures, which can be hollow and nearly 50% taller than the initial droplet, are formed during a growth stage. Although the formation of superficially similar structures has been explained for glass-forming polymers using a skin-buckling model which predicts the droplet to have constant surface area during the growth stage (L. Pauchard and C. Allain, Europhys. Lett., 2003, 62, 897-903), we demonstrate that this model is not applicable here as the surface area is shown to increase during growth for all $c_0$. We interpret our experimental data using a proposed drying and deposition process comprising the four stages: pinned drying; receding contact line; bootstrap growth, during which the liquid droplet is lifted upon freshly-precipitated solid; and late drying. Additional predictions of our model, including a criterion for predicting whether a conical structure will form, compare favourably with observations. We discuss how the specific chemical and physical properties of PEO, in particular its amphiphilic nature, its tendency to form crystalline spherulites rather than an amorphous glass at high concentrations and its anomalous surface tension values for MW = 100 kDa may be critical to the observed drying process.